Hello, The rash you have developed following Xolair injection may be a adverse drug reaction to Xolair. Rash with blisters, arthralgia or joint pains and fever are reported with the use of Xolair injection in few patients. The following are few treatment options: 1) Stop taking further doses of injection and report it to your prescribing physician. 2) Mild rash will resolve by itself with stoppage of drug. 3) Antihistamines like diphenhydramine can be taken orally once daily for 3 days. 4)If the rash is very confluent and irritating, then topical steroid creams like 1% hydrocortisone cream can be applied over the rash. 5) If there is airway swelling and difficulty in breathing, it might be anaphylaxis and requires immediate treatment with epinephrine. If your rash doesn t resolve with antihistamine and a topical steroid, you may need to consult a dermatologist for further evaluation. Take care. Hope I have answered your question. Let me know if I can assist you further. Regards, Dr. Pallavi M., Anesthesiologist